8026369: javac potentially ambiguous overload warning needs an improved scheme #12645

archiecobbs · 2023-02-19T23:52:52Z

This bug relates to the "potentially ambiguous overload" warning which is enabled by -Xlint:overloads.

The warning detects certain ambiguities that can cause problems for lambdas. For example, consider the interface Spliterator.OfInt, which declares these two methods:

void forEachRemaining(Consumer<? super Integer> action);
void forEachRemaining(IntConsumer action);

Both methods have the same name, same number of parameters, and take a lambda with the same "shape" in the same argument position. This causes an ambiguity in any code that wants to do this:

    spliterator.forEachRemaining(x -> { ... });

That code won't compile; instead, you'll get this error:

Ambiguity.java:4: error: reference to forEachRemaining is ambiguous
        spliterator.forEachRemaining(x -> { });
                   ^
  both method forEachRemaining(IntConsumer) in OfInt and method forEachRemaining(Consumer<? super Integer>) in OfInt match

The problem reported by the bug is that the warning fails to detect ambiguities which are created purely by inheritance, for example:

interface ConsumerOfInteger {
    void foo(Consumer<Integer> c);
}

interface IntegerConsumer {
    void foo(IntConsumer c);
}

// We should get a warning here...
interface Test extends ConsumerOfInteger, IntegerConsumer {
}

The cause of the bug is that ambiguities are detected on a per-method basis, by checking whether a method is part of an ambiguity pair when we visit that method. So if the methods in an ambiguity pair are inherited from two distinct supertypes, we'll miss the ambiguity.

To fix the problem, we need to look for ambiguities on a per-class level, checking all pairs of methods. However, it's not that simple - we only want to "blame" a class when that class itself, and not some supertype, is responsible for creating the ambiguity. For example, any interface extending Spliterator.OfInt will automatically inherit the two ambiguities mentioned above, but these are not the interface's fault so to speak so no warning should be generated. Making things more complicated is the fact that methods can be overridden and declared in generic classes so they only conflict in some subtypes, etc.

So we generate the warning when there are two methods m1 and m2 in a class C such that:

m1 and m2 consitiute a "potentially ambiguous overload" (using the same definition as before)
There is no direct supertype T of C such that m1 and m2, or some methods they override, both exist in T and constitute a "potentially ambiguous overload" as members of T
We haven't already generated a warning for either m1 or m2 in class C

If either method is declared in C, we locate the warning there, but when both methods are inherited, there's no method declaration to point at so the warning is instead located at the class declaration.

I noticed a couple of other minor bugs; these are also being fixed here:

(1) For inherited methods, the method signatures were being reported as they are declared, rather than in the context of the class being visited. As a result, when a methods is inherited from a generic supertype, the ambiguity is less clear. Here's an example:

interface Upper<T> {
    void foo(T c);
}

interface Lower extends Upper<IntConsumer> {
    void foo(Consumer<Integer> c);
}

Currently, the error is reported as:

warning: [overloads] foo(Consumer<Integer>) in Lower is potentially ambiguous with foo(T) in Upper

Reporting the method signatures in the context of the class being visited makes the ambiguity clearer:

warning: [overloads] foo(Consumer<Integer>) in Lower is potentially ambiguous with foo(IntConsumer) in Upper

(2) When a method is identified as part of an ambiguous pair, we were setting a POTENTIALLY_AMBIGUOUS flag on it. This caused it to be forever excluded from future warnings. For methods that are declared in the class we're visiting, this makes sense, but it doesn't make sense for inherited methods, because it disqualifies them from participating in the analysis of any other class that also inherits them.

As a result, for a class like the one below, the compiler was only generating one warning instead of three:

public interface SuperIface {
    void foo(Consumer<Integer> c);
}

public interface I1 extends SuperIface { 
    void foo(IntConsumer c);        // warning was generated here
}   

public interface I2 extends SuperIface { 
    void foo(IntConsumer c);        // no warning was generated here
}   

public interface I3 extends SuperIface { 
    void foo(IntConsumer c);        // no warning was generated here
}

With this patch the POTENTIALLY_AMBIGUOUS flag is no longer needed. I wasn't sure whether to renumber all the subsequent flags, or just leave an empty placeholder, so I chose the latter.

Finally, this fix uncovers new warnings in java.base and java.desktop, so these are now suppressed in the patch.

Progress

Change must be properly reviewed (1 review required, with at least 1 Reviewer)
Change must not contain extraneous whitespace
Commit message must refer to an issue
Change requires CSR request JDK-8303170 to be approved

Issues

JDK-8026369: javac potentially ambiguous overload warning needs an improved scheme
JDK-8303170: Generate "potentially ambiguous overload" for two inherited methods (CSR)

Reviewers

Vicente Romero (@vicente-romero-oracle - Reviewer)

Reviewing

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bridgekeeper · 2023-02-19T23:53:52Z

👋 Welcome back archiecobbs! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2023-02-19T23:55:23Z

@archiecobbs The following labels will be automatically applied to this pull request:

client
compiler
core-libs
i18n

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mlbridge · 2023-02-19T23:59:12Z

Webrevs

SWinxy · 2023-02-20T23:53:05Z

In the AWTEventMulticaster class(es), which interfaces are causing the ambiguity?

… not needed.

archiecobbs · 2023-02-21T01:36:29Z

In the AWTEventMulticaster class(es), which interfaces are causing the ambiguity?

Ah - my apologies. Those got added during development but were not needed once all the bugs were worked out.

I've removed them and that will also eliminate client and i18n from review. Thanks!

/label remove client,i18n

openjdk · 2023-02-21T01:38:29Z

@archiecobbs
The client label was successfully removed.

The i18n label was successfully removed.

vicente-romero-oracle · 2023-02-22T04:22:32Z

src/jdk.compiler/share/classes/com/sun/tools/javac/code/Flags.java

     */
-    public static final long POTENTIALLY_AMBIGUOUS = 1L<<48;
+    public static final long UNUSED_1 = 1L<<48;


what about just adding the comment saying that this spot is free without declaring an unused flag?

Works for me - as long as it's OK that this will cause the Flags enum constant ordinals to renumber after that point.

Fixed in 0131b9b.

vicente-romero-oracle · 2023-02-24T11:49:55Z

I think a CSR is needed as we will be generating more warnings and projects compiling with -Werror could see compilation errors

archiecobbs · 2023-02-24T15:56:05Z

I think a CSR is needed as we will be generating more warnings and projects compiling with -Werror could see compilation errors

Will do - thanks.

vicente-romero-oracle · 2023-02-24T18:01:56Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

+     *  ambiguities for both forEachRemaining() and tryAdvance() (in both cases the
+     *  overloads are IntConsumer and Consumer&lt;? super Integer&gt;). So we only want
+     *  to "blame" a class when that class is itself responsible for creating the
+     *  ambiguity. So we declare that site is "responsible" for the ambigutity between


typo: ambigutity -> ambiguity

Thanks - fixed.

vicente-romero-oracle · 2023-02-24T19:00:51Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

+     *  As an optimization, we first check if either method is declared in site and
+     *  does not override any other methods (in which case site is responsible).
+     */
+    void checkPotentiallyAmbiguousOverloads(JCClassDecl tree, Type site) {


general comments: overall looks correct but I think the code should be split a bit using helper methods, that will help with readability, I think. Side: I'm a bit worried that overuse of streams in this code could imply some performance hit. Of course if the corresponding lint warning is not enabled we will skip it but a lot of projects compile with -Xlint:all nowadays.

I think the code should be split a bit using helper methods

OK - will do.

I'm a bit worried that overuse of streams in this code could imply some performance hit

I asked basically the same question (in a separate thread) two days ago and nobody replied with a definitive answer (not surprising).

However, note also that in that same thread Christoph reported no timing difference between Stream vs. for() loop (here), although there were more allocations. FWIW.

Sorry to go off on a tangent here, but I'm sure this issue will come up again and it would be nice to have some kind of (informal) policy to go on...

I generally try to follow the "measure first, optimize second" rule to avoid preemptive "optimizations" that come at the expense of code clarity for unproven meaningful benefit.

So I can de-Stream the code but are we sure it's worth it? Are we going to have a no Stream policy in the compiler code? Why did we develop Stream's if they can't be used in a mainstream tool like javac? Where does the madness end? :)

There is also the larger philosophical question as well, which is that if a Stream is appreciably slower than the semantically-equivalent for() loop, then isn't that a Hotspot problem, not a developer problem?

I'm not saying that we should de-stream the code, actually we can do that later on, in a separate issue, iff there is a performance related complain, but it is true that in the past, I have seen some performance issues and the final culprit have been streams. But you are right it could be that it is not worthy to affect the readability of the code.

OK thanks, I'll leave it for now - but it would be nice to (someday) do some comprehensive testing so we have a better intuitive understanding of the performance impact of using a Stream in any particular situation.

I wonder if there is some IDE tool that could automatically Streamify and/or de-Streamify loops. If so, we could apply it to the entire compiler and compare...

yes that would be nice to have, although streams have different effects depending on how hot the code path is, so in general they are OK. Inference code is one of the places where I have seen that using streams is not a good idea for example

vicente-romero-oracle · 2023-02-24T22:24:10Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

+        List<java.util.List<MethodSymbol>> methodsByName = StreamSupport.stream(
+            types.membersClosure(site, false).getSymbols(new ClashFilter(site), RECURSIVE).spliterator(), false)
+          .map(MethodSymbol.class::cast)
+          .filter(m -> m.owner.type.tsym != syms.objectType.tsym)


nit: this filter could be folded with the one above, new ClashFilter(site)

Should be fixed in 7d8a54c.

vicente-romero-oracle · 2023-02-24T22:25:11Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

+                // Gather all supertype methods overridden by m, directly or indirectly
+                java.util.List<MethodSymbol> overriddenMethods = list.stream()
+                  .filter(m2 -> m2 != m)
+                  .filter(m2 -> overrides.test(m, m2))


can these two filters be folded?

Should be fixed in 7d8a54c.

vicente-romero-oracle · 2023-02-25T04:05:29Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

+          .stream()
+          .sorted(Comparator.comparing(e -> e.getKey().toString()))
+          .map(Map.Entry::getValue)
+          .peek(Collections::reverse)               // seems to help warning ordering


not sure about reversing the order here, it seems to me that if the order is reversed then the warning is shown in the first occurrence of a method instead of in the second which is the offending one. So for example for this code:

interface I1 { void foo(Consumer<Integer> c); void foo(IntConsumer c); }

the warning is shown for the first method when I think it should be shown for the second which is the one introducing the ambiguity

EDIT, after your last commit, this comment now applies to: methodGroups.forEach(Collections::reverse);

The reversing was originally done in order to maintain consistency with the existing regression test outputs. But you are right - it results in visiting the methods backwards.

Fixed in e25cece.

vicente-romero-oracle · 2023-02-25T13:38:05Z

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

+     *  As an optimization, we first check if either method is declared in C and does not override
+     *  any other methods; in this case the class is definitely responsible.
+     */
+    BiPredicate<MethodSymbol, MethodSymbol> buildResponsiblePredicate(Type site,


this method not only builds the predicate as its name indicates, it also removes methods from the methodGroups, not saying that we should split this method but I think that this removing activity should be mentioned in the comment above

Oops, my bad - previous refactoring was incomplete.

Should be fixed in e25cece.

Thanks again for the careful review!

sure, thanks, nice refactoring

…incomplete.

vicente-romero-oracle

looks good, thanks for fixing this!

openjdk · 2023-02-27T21:35:31Z

@archiecobbs This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8026369: javac potentially ambiguous overload warning needs an improved scheme

Reviewed-by: vromero

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 122 new commits pushed to the master branch:

f7f1036: 8303068: Memory leak in DwarfFile::LineNumberProgram::run_line_number_program
784f7b1: 8293667: Align jlink's --compress option with jmod's --compress option
54603aa: 8303208: JFR: 'jfr print' displays incorrect timestamps
4c169d2: 8303253: Remove unnecessary calls to super() in java.time value based classes
b527edd: 8292914: Lambda proxies have unstable names
42330d2: 8029370: (fc) FileChannel javadoc not clear for cases where position == size
a253b46: 8301119: Support for GB18030-2022
55e6bb6: 8302685: Some javac unit tests aren't reliably closing open files
f5a1276: 8262895: [macos_aarch64] runtime/CompressedOops/CompressedClassPointers.java fails with 'Narrow klass base: 0x0000000000000000' missing from stdout/stderr
2fe4e5f: 8303169: Remove Windows specific workaround from libdt
... and 112 more: https://git.openjdk.org/jdk/compare/7abe26935ab4356de54acee93390a0d8be1ea289...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@vicente-romero-oracle) but any other Committer may sponsor as well.

➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

archiecobbs · 2023-02-27T21:42:48Z

/integrate

openjdk · 2023-02-27T21:44:57Z

@archiecobbs
Your change (at version e25cece) is now ready to be sponsored by a Committer.

vicente-romero-oracle · 2023-02-28T03:31:37Z

/sponsor

openjdk · 2023-02-28T03:33:20Z

Going to push as commit 1e3c9fd.
Since your change was applied there have been 124 commits pushed to the master branch:

14a014d: 8302124: HotSpot Style Guide should permit noreturn attribute
bca60f4: 8303249: JFR: Incorrect description of dumponexit
f7f1036: 8303068: Memory leak in DwarfFile::LineNumberProgram::run_line_number_program
784f7b1: 8293667: Align jlink's --compress option with jmod's --compress option
54603aa: 8303208: JFR: 'jfr print' displays incorrect timestamps
4c169d2: 8303253: Remove unnecessary calls to super() in java.time value based classes
b527edd: 8292914: Lambda proxies have unstable names
42330d2: 8029370: (fc) FileChannel javadoc not clear for cases where position == size
a253b46: 8301119: Support for GB18030-2022
55e6bb6: 8302685: Some javac unit tests aren't reliably closing open files
... and 114 more: https://git.openjdk.org/jdk/compare/7abe26935ab4356de54acee93390a0d8be1ea289...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-02-28T03:33:32Z

@vicente-romero-oracle @archiecobbs Pushed as commit 1e3c9fd.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

Fix incomplete detection of potentially ambiguous method declarations.

591e220

openjdk bot added the rfr Pull request is ready for review label Feb 19, 2023

openjdk bot added client client-libs-dev@openjdk.org core-libs core-libs-dev@openjdk.org compiler compiler-dev@openjdk.org i18n i18n-dev@openjdk.org labels Feb 19, 2023

Remove @SuppressWarnings("overloads") annotations that were added but…

88a5f99

… not needed.

openjdk bot removed client client-libs-dev@openjdk.org i18n i18n-dev@openjdk.org labels Feb 21, 2023

vicente-romero-oracle reviewed Feb 24, 2023

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Remove placeholder flag UNUSED_1; just leave a comment instead.

0131b9b

openjdk bot added the csr Pull request needs approved CSR before integration label Feb 24, 2023

vicente-romero-oracle reviewed Feb 24, 2023

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Fix typo in one comment and clarify another.

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vicente-romero-oracle reviewed Feb 24, 2023

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Do some refactoring & cleanup suggested in reviews.

7d8a54c

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archiecobbs added 2 commits February 25, 2023 10:01

Clean up previous buildResponsiblePredicate() refactoring, which was …

69cdc6d

…incomplete.

Process methods in order so the second method triggers the warning.

e25cece

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8026369: javac potentially ambiguous overload warning needs an improved scheme #12645

8026369: javac potentially ambiguous overload warning needs an improved scheme #12645

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8026369: javac potentially ambiguous overload warning needs an improved scheme #12645

8026369: javac potentially ambiguous overload warning needs an improved scheme #12645

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